Electrical resistor, conductor, and the like



Patented Jan. 7, 1930 PATENT OFFICE JAMES KELLEHER, OF CHIPIPAWA, ONTARIO, CANADA, ASSIGNOR TO HARPER ELEC- TRIC FURNACE CORPORATION, A CORPORATION OF NEW YORK ELECTRICAL RESISTOR, CONDUCTOR, AND THE LIKE No Drawing.

This invention relates'to carbonaceous resistors, conductors and similar articles which are designed to be heated, and more particularly it relates to an improved means for preventing the oxidation of such articles when heated, and to an improved method of treating such articles so as to protect them from oxidation.

The temperature at which oxidation occurs depends upon the form or composition of the carbonaceous material and also on the form in which the oxygen is permitted to reach such material. Amorphous carbon burns or oxidizes at a lower temperature than graphite and pure uncombined oxygen attacks such material more readily than when the oxygen iscombined, as with a metal in the form of an oxide. The present invention relates more specifically to protection from oxidation of .articles of amorphous carbon or graphite such as electric furnace terminals, electrodes, resistance elements, etc. by oxygen contained in the atmosphere surrounding such articles.

One embodiment of my invention relates to the protection of an electrode or heating element of a resistance furnace which may consist of a slab or rod of carbonaceous material of such proportion, shape and size as to present a high resistance to the passage of current and thereby generate heat when current is forced through it. Such resistors have, in the past, been enclosed in air-tight chambers of special design and expensive construction so as to protect them from oxidation, which takes place readily in an atmosphere containing even a trace of oxygen at temperatures of 2000 to 3000 C., and more readily in such an atmosphere at higher temperature. In accordance with such embodimentof my invention, I so treat the resistor surface that oxygen cannot come into contact with the carbon from the surounding gases or atmosphere.

The resistor is normally suspended between two currentcarrying terminals and the major portion of the resistor is subjected to the effects of the atmosphere in which itis immersed; This portion of the resistor I cover with a thin film or wash of silicon carbide, either in amorphous or crystalline form, such as finely divided ground silicon carbide Application filed July 17, 1925. Serial No. 44,182.

crystals, and mixed with this powder is some readily fusible salt such as borax, the amount of salt depending upon the temperature at which the resistor is to be run.

As the resistor is heated by the passage of current, the borax melts at about 900 C. and bonds the grains of silicon carbide together, holding them to the carbon material of the resistor, and also forms an air-tight film between each and every grain so as to prevent air penetrating to the carbon. As the temperature of the resistor is increased, some of the borax is reduced and the resultant sodium metal vapors and boron vapor find their way to the outer surface of the coating, as does also some unreduced borax. Here the vapors and unreduced borax come in contact with the air the vapors oxidizing to forma sodium oxide and a boron oxide, and this together with the borax forms a thin film through which the atmospheric oxygen cannot penetrate to the resistor proper. Between this film and the carbon is left an inert layer of silicon. carbide. While resistors have previously been coated for the purpose of protecting them from oxidation, such coatings have in the past been hard and dense and com posed of such material as clays. I find, however, that my improved coating remains in a semi-plastic state and this permits the expansion and contraction of the resistor when heating and cooling, and so adds considerably to the life of the resistor. When the resistor is designed for low temperatures, I may use as high as 30% by weight of borax to silicon carbide, and for resistances designed for hi h temperatures I may use as low as 5% by weight of borax to silicon carbide powder.

Another method of applying the coating to resistors, and especially where large surfaces have to be covered, is to impregnate the resistor surface with silicon vapor to a depth of .01 to .05 inch at a high temperature, thus forming a thin coating of silicon carbide which is in intimate contact with the carbon and adheres well to it, and then giving this bonaceous terminals of furnaces, furnace eleccon carbide and borax, and heating the coated article in air until the coating formed comprises an inner portion composed largely of silicon carbide and an outer portion containing oxidized boron.

high temperatures, a layer of silicon carbide covering said core, an outer nonoxidizable coating formed by fusing together borax and silicon carbide, said coating being adapted to be semi-plastic at high temperatures.

JAMES KELLEHER.

3. A resistor comprising the combination with a core of carbonaceous material, of a coating of silicon carbide and a fusible salt fused together as a protective surface layer which becomes semi-plastic at high temperatures.

4. A'resistor comprising the combination with a core of carbonaceous material, bf a protective coating having an outer surface film containing metal 'oxide and an intermediate inert layer of silicon carbide between said film and the carbonaceous material.

5. The method of preparing a carbona- I ceous resistor which consists in washing it with a solution of borax containing silicon carbide powder wherein the borax is present to from 5 to 30% to the silicon carbide powder, and electrically heating the resistor so coated.

6. The method asset forth in claim 5 wherein the resistor before washing is im-' pregnated at a high temperature with silicon vapor to a depth of .01 to .05 inch.

7. The method as set forth in clalm 5 wherein the resistorbefore washing is sub jected to the, action of silicon vapor at a high temperature.

8. The method offorming a protective coating'on an electrical resistor of carbonaj ceous material adapted to be operated at a high temperature comprising impregnating said carbonaceous material with silicon vapor to a depth of approximately .01 to .05 inch,

washing the= resistor with a solution of borax containing silicon carbide powder wherein the borax is present to about 5 to 30 per cent of the silicon carbide powder, and passing current through the resistor until a glaze is formed at the surface thereof and an intermediate layer of silicon carbide is left between the resistor and the surface glaze.

.9. A resistor comprising a core of carbon of low resistance and readily oizidizable at 

